Modern televisions with LED backlights have tended towards longer strings of serially connected LEDs that are controlled as one circuit. These strings, containing from 10 to 40 LEDs, can require voltages between 30 and 150 volts in order to operate effectively. In such strings, the current is regulated in order to control the color of the individual LEDs—more current tends to yield a cooler color of white, while less current tends to yield a warmer color. In order to control the color spectrum of the output, the current is held constant while the brightness is controlled using a PWM on/off mechanism—more time on yields a brighter screen
The current through these strings is typically controlled by a low voltage, high power current sink connected from the cathode of the LED string to ground. The high power current sink is cycled on and off at a given PWM duty cycle by a controller within the television, resulting in a brighter or darker screen. Such a basic circuit is shown in FIG. 1.
A problem arises when the high power current sink cycles off and the current through the LED string 102 stops flowing. When this occurs, the forward voltages of the individual LEDs in the string each shrink to zero, and the forward voltage of the serially connected LED string also approaches zero. As a result, the high power current sink 104 and the integrated circuit that contains and/or controls the high power current sink 104 are presented with most of the voltage of the voltage source required to drive the LED string. This high voltage can damage the high power current sink 104 and the integrated circuit.
To protect this circuitry, which is generally comprised of devices with lower voltage ratings than the LED supply voltage, current implementations of these circuits use a device with a high voltage rating inserted in series between the cathode of the series LEDs 202 and the low voltage current sink 206; such a configuration is shown in FIG. 2.
In this figure, the voltage Vgate must be greater than the dropout voltage of the current sink plus the high voltage MOSFET 204 threshold voltage. The voltage of Vgate must be less than the voltage rating of the current sink plus the high voltage MOSFET 204 threshold voltage. Vgate is typically a fixed supply. 24 v is a typical supply available in TV systems and can be used as Vgate either directly or voltage divided down using a resistor divider.
FIG. 3 shows this prior art as a 6 channel integrated circuit 302. In this drawing, the MOSFETs depicted in the lower, descending row are discrete components of the high power current sinks 304; in this implementation, they have been removed from the primary IC in order to aid in heat dissipation. The MOSFETs depicted in the upper row are the high voltage MOSFETs 306 that protect both the circuitry of the integrated circuit as well as the high power current sink MOSFETs 304 from voltage of the power supply.
The external, high voltage MOSFETs 306 present a few disadvantages. The devices add cost, both the actual cost of the device and the cost of the implementation of the device within the television or any other enclosing appliance. In addition, the MOSFETs also add wasteful power dissipation to the system, as the current used to power the LED strings must flow through the high voltage device (and its associated voltage drop) on its way to the high power current sink.